U.S. Pat. Nos. 5,552,261 and 5,529,473 disclose using the diffusion of monomers which have an increased or decreased refractive index compared to a surrounding liquid matrix for generation of a refractive index gradient. This effect, which is known as the Colburn-Haines effect for polymers, can lead after subsequent polymerization to a product with refractive index gradients. Such polymers are also referred to as photopolymers. To produce the refractive index gradient, a local polymerization is induced, which leads to a potential difference. This promotes the diffusion of further monomers to the site of local polymerization. This alters the refractive index at this point compared to the direct environment.
The generation of sharp refractive index gradients by the Colburn-Haines effect therefore requires not only the efficient diffusion of the monomers but also rapid and efficient depletion of the monomers. The matrix is usually a polymer which, for example, is dissolved in a solvent. It is important that the reaction of the monomers must not influence the matrix. Moreover, the formation of the refractive index gradient should not lead to a change in the overall material, for example as the result of shrinkage. This is important especially in the generation of refractive index gradients in very thick layers.
One problem in the generation of refractive index gradients on the basis of purely organic systems is the limited range of refractive indices achievable. One means of increasing the range is the use of inorganic components.
For example, in application US 2005/0101698, a concentration gradient of nanoparticles is generated in a composite material. This process allows the production of volume holograms with a refraction efficiency of 90%. However, the thickness of the layers is limited and the material exhibits high shrinkage due to the free-radical polymerization. Moreover, the slow diffusion of the nanoparticles limits the possible refractive indices and useable matrices.
In summary, it can be stated that the development of photopolymers has recorded great advances in the last few years. Nevertheless, the known systems still have some disadvantages. For instance, the systems used are not sensitive enough to enable very sharp modulation of the refractive index. The sensitivity includes the light intensity required for polymerization, and also the exposure time required for production of the gradient structure. Both limit resolution and refraction efficiency in the gradient structures produced. At the same time, the range of refractive index modulation achieved in the material also plays an important role. Both parameters mentioned, for example, limit the minimum layer thickness of the material with which the production of optical gradient structures is still possible.
Since optical gradient structures are nowadays used in many fields with very different requirements, for example with regard to mechanical flexibility, thickness and stability, high variability of the components used is of great significance.
It is an object of the invention to provide a simple, universally applicable and inexpensive process for producing optical elements with gradient structure, which overcomes the stated disadvantages of the prior art.